doc/encoder: add the missing qsv encoders

Reviewed-by: Gyan Doshi <ffmpeg@gyani.pro>
Signed-off-by: Zhong Li <zhongli_dev@126.com>

doc/encoders.texi

@@ -2776,7 +2776,7 @@ recommended value) and do not set a size constraint.
 
 @section QSV encoders
 
-The family of Intel QuickSync Video encoders (MPEG-2, H.264 and HEVC)
+The family of Intel QuickSync Video encoders (MPEG-2, H.264, HEVC, JPEG/MJPEG and VP9)
 
 The ratecontrol method is selected as follows: